Directly heated alternating current cathode radio tube



May 23, 1933. w. J. KIMMELL El AL 1,911,024

DIRECTLY HEATED ALTERNA TING' CURRENT CATHODE RADIO TUBE Filed Aug. 3,1927 INVENTORS W/'///'0m J. Kim me// and Lee Sufher/in ATTORNEY PatentedMay 23, I933 UNITED STATESPATENT OFFICE WILIIAI J. mm, OF TBAIFORID, ANDLEE SUTHERLIJT, 01' EAST PITTSBURGH, PENNSYLVANIA, ASSIGNOR TOWETINGHOUSE ELECTRIC AND MANUFACTURING COMPANY, A CORPORATION OFPENNSY'LVANIA DIRECTLY HEATED ALTERNATING CURRENT CATHODIJ RADIO TUBEApplication filed August a, 1927. semi No. 210,262.

. be heated by alternating current of the frequency'commonly suppliedfor house lighting without producing an objectionable hum in telephonereceivers or loud speakers supplied thereby.

Another object of our invention is to produe a form of cathode adaptedfor radio tubesof the kind above described which may be-h'eated byalternating current of commercial frequency without the production ofany objectionable bum in apparatus connect-- ed thereto.

A further object of our lIlVGIltlOIl' is to devise a way to proportion acathode of a filamentary type to be used in radio tubes of the kinddescribed above in such a way that it may be heated by alternatingcurrent of audible frequency without the production of objectionablehumin the apparatus connected thereto.

Other objects vof our invention will be apparent through reading thefollowing speci cation taken in conection with the accompanying drawing,in which Figure 1 shows a radio tube embodying our invention.

Fig. 2 is a diagram which will be used to explain the propertiesthereof.

The ordinary radio receiving or amplifying tube of the prior artcomprises a cylindrical anode, a grid electrode surrounded thereby and,near the center of the cylinder, a cathode comprising a filament adaptedto be heated to a temperature sufiiciently high to emit electronsfreely. The ordinary way of heating this filament'has been to supply itwith current from a battery or other direct-current source.

Considered as sources of electric power, batteries are relativelyexpensive devices and it would have been desirable at all times.

to have heated the filament by current. from ordinaryhous'e lightingmains. However, a

very large proportion of house lighting services are of the alternatingcurrent type and when it was attem ted to use such sources of supply forthe lament, an objectionable hum of a harmonic frequencyof thealternating current was produced in instruments such as telephones orloud speakers connected in the plate circuits of the radio tubes. It hasbeen the object of the experiments which we have carried out to discoverthe cause of this hum and to devise ways by which it might beeliminated.

As the result of the analysis which we have made of this subject we havefound that a number of separate causes contribute in greater or lessdegree to the production of positive at any par-' as we will hereinaftershow produces'fluctuations in the current from the anode.

.3. The alternating magnetic field set up by the alternating currentthrough the filament, which field deflects the electrons from thestraight path which they would otherwise follow' between the cathode andthe anode.

4. The variations in temperature of the filament, resulting from therise and fall of the alternating heating current in its cycle.

In receiving tubes of the many types, one or more of these causes mayproduce efiects in the anode circuit. so small as not to be of practicalimportance. It is probably only the two effects first mentioned whichproduce the hum actually heard in the case of ordinary receiving tubes.We have dis,- covered that the instantaneous effects of the first andthird factors listed are opposite in kind to those roduced by thesecond, so that .the latter ten s to increase the anode current at thetime each of the former tends to decrease it. The effect of the fourth,while a periodic quantity of the same frequency as the first and third,is probably dephased therefrom by nearly three quarters of a cycle so itcannot be exactly neutralized by either of the first three. Except forvery thin filaments it is neli 'ble in magnitude however.

We have furt er found that by pro erly proportioning the diameter,length, s ape,v

resistivity, and temperature of operation of the filament, it ispossible to make the first two effects substantially equal as well asopposite to each other, from which it follows that the net eflect of allis made zero.

While it is possible to embody the broad principles of our discovery, inmany different structures, we show herewith a tube which makes use ofthem in a cathode which is similar to certain cathodes of prior artexcept in respect to this proper proportioning between its variousdimensions.

-Referring in detail to Fig. 1 which illus trates an embodiment of ourinvention, an evacuated container 1 of the conventional type has a press2 upon which the in-leading wires support a cylindrical anode 3, aid 4and a cathode filament 5 which speci cally embodies the features of ourinvention as above outlined. The anode 3 and grid 4 are of conventionaltype and need no se arate descri tion. The cathode consists of a V-shape filament having a length of approximately 5.1 cms. and the ends ofthe V being approximately 1.0 cms. apart. This filament comprises a baseof nickel approximately .0056X .0325 mms. upon which 1s deposited acoating comprising a mixture of barium oxide and strontium oxide,approximatel 0.002 mms. thick. Cathodes coated in thls way are wellknown in the art under the term oxide coated cathodes. Except for itsrelative proportions therefore a tube of our invention resembles closelythose known to the prior art.

The resistance when hot, of this cathode.

is about 1.4 ohms and it operates with about 1.5 volts impressed acrossits terminals.

The cathode is positioned in the middle plane of the anode and is s acedfrom it by approximately 2.2 mm.. he grid may be positoned about midwaybetween cathode and anode. Such a tube is designed to operate with a 135volt B-battery between cathode I and anode.

Turmng to the various causes of hum enumerated above, their efiects maybe analyzed in connection with the diagram of Fig. 2, which may beconsidered as a view.

looking down on the anode and cathode shown in Fig. 1. The midoint ofthe cathode is taken as the axis 0 coordinates, and distances to theright of the point are represented by the quantity +X while dlS- tancesto the left thereof, are represented by X. The two ends of the filamentare located at the distance +L and L respectively. Between the mid-pointof the cathode and the plate, the B-Battery, having constantelectromotive force e,, is connected. Between the two ends or thecathode is impressed the electromotive force e=E sin pt. Considering thefirst cause of hum listed above, there will exist at the silrface of thecathode filament, an electric force due to thevoltage between oppositeends of the filament. This force will be .uniform all along the filamentdue to the fact that both the distance and the potential differencebetween points opposite each other on the two sides of the V decrease atthe same rate as the apex of the V is approached. This force may as afirst approximation be considered to side-track certain electronsemitted from the cathode which otherwise would flow to the anode.Because the voltage drop alongethe filament is small compared withaforesaid. The number which will be left available for flow to the anodewhen the voltage E sin pt is impressed on the cathode may be expressedas KE 2 8111 pt N-N,,(1

Where E sin it is IR drop between ends of the cathode 2 is the distanceseparating the ends of the cathode filament and K is a constant. Turningto the second cause of hum, the current flowing to the anode from thecathode-may be calculated with the aid of Fig. 2 by the use of the wellknown Langmulr formula.

' i, Nl e Where i, is current from a hot filament of length l to a radiotube anode, e is the potential difi'erence between the cathode and theanode and N is proportional to the number of electrons available at thecathode.

In the case of a three-electrode tube the Langmuir formula takes'theform where E, is anode volta e; E, is grid voltage; and ,u is the amplication constant of the tube; see page 417 of Principles of RadioCommunication by J. H. Morecroft 1921, Wylie and Sons, New York,publishers.

Applying the foregoing formula to current between filament and anode inFig. 2, the potential difference betweenthe anode and any point on thecathode at a distance from the mid-point thereof is represented bye=e,,+% E sin pt In accordance with the Langmuir formula, the currentflowing to the plate from a small element da: will be represented by Inordinary receiving tubes, the filament voltage E is small compared toep. Hence, this expression may be expanded and written Substituting forNe value as given above and dropping the terms of powers higher than thesecond of r gives a Upon integration, this gives for the total anodecurrent L E K E 2 1 )(1 cos pt) This shows that the anode currentconsists of direct current component current which would flow if theB-battery alone were supplying power to the circuit.

}cos 2 pt L E K E i 32 e 4L is zero the alternating current componentproducing hum disappears.

Accordingly, we have conceived that it is possible to build a tube soproportioned that the quantity in parenthesis shall be zero and that nohum will be produced in such a tube.

' For instance by varying L, the distance sepaparagraph above reduces toIt will be evident that a variation of L is not the only way ofeliminating hum by making the quantity in the parenthesis zero. Thus,since the ratio of E to 6,, is not the same in the two terms, avariation of this ratio may be used to make the two terms equal. Thus,by varying the ratio of B-battery voltage to filament voltage, thedesired adjustment can be attained.

In most cases, the third factor enumerated as cause of hum is ofnegligible effect; but in any case, Where its effects are appreciable,it is possible to counteract them, also by increasing or decreasing thesecond term in the parenthesis, as by varying L. That is to say L shouldbe decreased to counteract hum if the magnetic field is appreciable.

It will also be evident that where the third cause of hum enumerated isnot negligible, to increase or decrease its value bychanging themagnitude of the cathode heating current is a possible way to cause theopposing factors in hum to balance.-\ Likewise by changing the specificresistivity of the filament material the voltage and current in thecathode may be varied, and the magnitude of the terms, entering into theforegoing equations thereby varied. Once they have become acquaintedwith the foregoing method of analyzing the factors entering into humproductions numerous other specific ways of varying the quantities tobalance the terms will be evident to those skilled in the art.

The final object to be attained by any of the foregoing methods is, ofcourse, that the effects of the various causes shall sum up to zero. Ourinvention broadly covers the elimination of hum by such an adjustment ofthe various factors which cause it.

While, in compliance with the-patent statutes, we have described oneembodiment of our invention in detail, and have disclosed ourunderstanding of the principles underlying it, our invention is notdependent upon the correctness of the assumptions underlying themathematical analysis here given, and wedo not wish to be limitedthereby, since its purpose was mainly to illustrate the relations ofvarious quantities and the method of using them in proportioning astructure embodying our invention. It will be evident to those skilledin the art that the broad principles which we have utilized may beapplied in connection with different specified embodiments, and wetherefore, desire that our claims shall be limited only by theirexplicit terms and in accordance with the prior art.

We claim as our invention: 1. In combination with an anode, a cathodeadapted to emit electrons, a source of direct current connected incircuit therewith, a

source of alternating current connected to heat said cathode,-saidcathode heating current being large enough so that the magnetic fluxproducedby it in the neighborhood of the filament has a substantialtendency to produce variations in the anode current, said cathode beingso proportioned that the fluctuations produced by the alternatingpotential drop along the cathode in the anode current are substantiallyequal to the fluctuations in said anode current due to cross flow ofelectrons between the different parts of the cathode and due to thealternating magnetic field set up by the alternating current through thecathode.

2. The method of operating an electrondischarge tube having a V-shapedfilament energized by alternating current and an anode connected througha direct current source to said V-shaped filament, said-alternatingcurrent being lar e enough so that the magnetic flux produced y it inthe neighborhood of the filament has a substantial tendency to producevariations in the anode-current, which comprises spreading the legs ofsaid V-shaped filament and spacing said filament from said anode thatthe effect on' the anode current because of the cross flow of electronsbetween portions of the filament and because of the alternating magneticfield set up by the alternating current through the filament isneutralized by the efiect on the anode current of the rise and fall ofpotential of various portions of the filament. with alternations of thecurrent therethrough.

In testimony whereof, we have hereunto subscribed our names this 26thday of July, 192 i WILLIAM J. KIMMELL. LEE SUTHERLIN.

